The 1968 Gun Control Act established 1898 (70 years before the Act) as the legal cut off date. Since then, ATF has established "fifty years old"--with some exceptions for out of production guns--as "Curio and Relic" more likely to be owned as a collectible than used as a weapon, with a formal list that includes moving some NFA (Title II) firearms (pre-1934 short barrel trappers, orginal C96 w. shoulder stock, etc., requiring Form 4) to GCA (Title I) status (ordinary rifle, shotgun or handgun, requiring Form 4473).
The closest to "antique" I shoot are:
o Mauser C96, ser no in the 107xxx range, probably 1912-1913.
o Webley .455 Mark IV, BP proofmarks VR Queen Victoria, pre 1902.
"shooting antiques" I could tell tales of scrounging items from a collapsed, abandoned shed from my grandparents' farm or from my wifes' grandparents' homestead for plinking targets, but I wouldn't want to give an antique picker a heart attack. (Or would I: "first we shot the hangy down crystal bobs from rim of the Tiffany lampshade, then we took turns calling out the color of the stained glass panes we'd shoot next")